The function of a fluorescent lamp ballast is to connect the lamp bulbs to the power line in a way that will provide just enough current to the bulbs to operate them at the desired brightness level. Since the bulbs have a negative resistance characteristic which is dependent on beam current, temperature, bulb type and age, and is very different during starting than during operation once started, this is not a simple task. Standard magnetic ballasts perform the current control function by inserting a relatively large iron core inductor between the bulbs and the source of power. This achieves the control function with adequate stability but to change the brightness level requires changing the inductance, which cannot be done in a simple manner based on a control knob position or control D.C. voltage. Electronic ballasts simulate the action of the inductor with switchmode regulator circuits using feedback techniques to control the voltage across the bulbs based on measured current through them. Over a wide brightness range the impedance of the bulbs changes so much that instability usually results, and the bulbs flicker, turn off, or get into an inefficient operating state where the special coating on the filaments quickly burns off, making the bulbs unusable.
Some current electronic ballasts deal with this problem by operating the bulbs at a relatively high frequency (above the normal audio range) and use inductors and capacitors to form a resonant circuit around the bulbs, with a resonant frequency at or near the frequency at which the bulbs are driven. Thus, as the bulb impedance changes, the "Q" of the resonant circuit changes with it, since the bulb is part of the resonant circuit, and more or less drive voltage is immediately available as required, even though the feedback control circuit might not be able to respond quickly enough if the resonant circuit were not there. This technique works well over a limited range of brightness, but to apply this technique over a wide brightness range, substantial changes in the inductance and capacitance forming the resonant circuit would be required, and there is no practical way to accomplish this.
The present invention uses a resonant circuit around each bulb or bulb set in a similar fashion, but instead of using a resonant frequency at or near the drive frequency, a resonant frequency much higher than the drive frequency is used, i.e. 130 KHz., and the drive frequency is selected to be an exact submultiple of the resonant frequency of the inductive and capacitive resonant circuit components around the bulb. Different submultiple frequencies are selected under different operating conditions, achieving many benefits which would be obtained by changing the inductance and capacitance values of the resonant circuit around the bulb if such changes were practical, which they are not.